new champs:
Kled - melee top laner, rides a mount, dismounts at low health, making him weaker but giving him the opportunity to heal for a ton by remounting
Ivern - support jungler, has a gimmick where he doesn't have to actively fight camps to clear them
Camille (upcoming) - diving fighter most similar to Irelia
Reworks:
Yorick - powerful pushing top laner, summons lots of mostly uncontrollable ghouls
Kog'maw - mostly un-reworked
Alistar - passive and E switched up and changed a bit
Assassin update:
Talon - now can parkour over walls
Leblanc - takes a bit longer to burst people, now does her mimic when she ults rather than at low health
Katarina - now leaves daggers on the ground, does a damage spin when she picks one up
Rengar - Q is now a two hit skillshot instead of an empowered auto. He also has Olaf and Ekko's ults on a basic ability. He's kind of busted.
Lots of smaller changes that aren't that important.
Preseason:
The jungle sucks now. You have to start buffs because Krugs and Gromp spawn later. Krugs and Raptors are super annoying to take. Start blue side if you can.
There are now "plants" in the jungle. You autoattack them to activate them. There are three kinds: one works like Zigg's Satchel Charge, one reveals a big area, and one leaves fruit on the ground that heal you when you walk over them.
AD Assassin items are now better for assassins and worse for everyone else.